Effect of coating parameters and heat treatment on the adhesive properties of electroless Ni-P coatings

The adhesive properties of coatings have pronounced effects on their quality. The effect of various coating parameters and post-deposition heat treatment on the adhesive properties of low and high phosphorus electroless nickel-phosphorus (EN) coatings was investigated using three-point bending tests. The critical load and mid-span deflection were used as measures of the adhesive strength of the coatings. A good correlation was found between the coating parameters and the critical load and deflection values at crack initiation. The critical load was found to increase with increasing coating thickness for EN-coated specimens tested in the as-deposited condition. The critical load and mid-span deflection of as-plated EN coatings increased with increasing phosphorus content. However, thick coatings cracked at lower deflection values than thin coatings. Post-deposition heat treatment at 400 degrees C for I hour adversely affected the measured indices due to increase in brittleness of the coatings occasioned by the crystallization of nickel crystals and nickel phosphide (NixPy) precipitates. Further, the critical load and deflection values at cracking were found to be dependent upon the mechanical properties of the substrate material.